The invention concerns a process for the determination of colorimetric differences between two screen pattern (half tone) fields, in particular two gray balance fields, printed by a printing machine, by the optical scanning of the screen fields and the evaluation of the reflected light.
The invention further relates to a process for the color control or ink regulation of the print of a printing machine, wherein measuring fields are optically detected on production sheets printed by the printing machine, in order to determine the color difference of the measuring field detected from a predetermined set color location and to produce a correction value from the color difference for the adjustment of the ink control elements of the printing machine, so that undesirable color deviations on the production sheets subsequently printed with the new ink control setting will become minimal.
Processes of this type are known from EP-A 228 347, DE-A1 36 26 423.7 and EP-A2 196 431.
Processes of the aforementioned type for the determination of colorimetric differences are used for quality evaluation and require the employment of colorimetric instruments or spectrophotometers in order to determine the coordinates associated with a half tone field, in particular a gray balance field, in a color space. The use of such instruments is expensive and complex in view of the extensive optical and electronic effort required. It is further known to carry out quality evaluations via measured densitometric values. While quality evaluations via a densitometric measuring system or densitometric parameters have the advantage that less expensive instruments, i.e., densitometers instead of spectrophotometers, may be used, densitometric values are not especially practical and are not equivalent to values obtained in true colorimetric systems. In the state of the art, use of densitometers restricts one to a densitometric measuring system that for quality evaluations is poorer than colorimetric numbers in a color space equidistant in perception, such as the L*a*b* color space or the LUV color space.
From EP-A 321 402 a process is known for the color control and ink regulation of a printing machine, in which via a spectrophotometer, measuring fields are scanned in order to obtain color coordinates in a colorimetric measuring system and to produce, by a coordinate comparison from the color difference of the measuring field being scanned relative to a predetermined set color position, a correction value for the adjustment of the ink control elements of the printing machine. This is effected in a manner such that a given set color position located outside a correction color space is replaced by an attainable set color position on the surface of a correction color space with a color difference from the given set color position, such that the components essential for print quality are minimized. The realization of such a control strategy requires an operation in a colorimetric coordinate system, for example the L*a*b* color space. The invention of EP-A 321 402, requires the use of a spectrophotometer in place of a densitometer.